The present invention relates to thermophotovoltaic energy conversion and, more particularly, to a method for thermophotovoltaic energy conversion using photonic bandgap selective emitters based on photonic crystals.
Thermophotovoltaic (TPV) energy conversion converts the radiant energy of a high-temperature source (xe2x80x9cemitterxe2x80x9d) directly into electricity using a photovoltaic cell. TPV has a number of attractive features for electricity generation, including fuel versatility (e.g., nuclear, fossil, or solar energy can be used to heat the emitter), quiet operation, low maintenance, low emissions, light weight, high power density, modularity, and cogeneration of heat and power. If TPV efficiencies can be improved, TPV could potentially be used for distributed power, automotive, military, and other applications wherever fuel cells, microturbines, or cogeneration are presently considered. See, e.g., T. J. Coutts and M. C. Fitzgerald, xe2x80x9cThermophotovoltaics,xe2x80x9d Scientific American, pp. 90-95, September 1998; and T. J. Coutts, xe2x80x9cA Review of Progress in Thermophotovoltaic Generation of Electricity,xe2x80x9d Renewable and Sustainable Energy Reviews 3, 77 (1999).
A TPV system can be conveniently described as comprising a heat source to heat an emitter (also known as a radiator) to a sufficiently high temperature to cause the emitter to emit in the visible or near-infrared region of the electromagnetic spectrum and an array of photovoltaic cells to collect the emission from the emitter and convert it into electricity. Whilst the concept of TPV energy conversion is not new, interest in TPV has grown due to recent advances in low-bandgap photovoltaic cells. Such low-bandgap photovoltaic cells are desired to work with emitters at manageable temperatures (e.g., up to 1800xc2x0 K).
The efficiency of current TPV systems could be significantly improved, if the spectrum of the emitter could be controlled to more precisely match the peak spectral response of the photovoltaic cell. In particular, photovoltaic cells have energy conversion efficiencies of over 50% if illuminated with monochromatic light near their electronic bandgap. Spectral control can be achieved by either tailoring the spectrum of the emitted radiation (i.e., so that the source behaves as a xe2x80x9cselective emitterxe2x80x9d) or by using spectral filters to transmit the matched spectrum and return unwanted radiation back to the emitter. Achieving high TPV efficiencies requires very high performance in terms of the spectral selectivity or reflectivity, bandwidth, and the angular distribution with either the selective emitter or the spectral filter.
Photonic crystals comprise materials having a periodic variation in dielectric constant on the order of the wavelength of light. The periodic variation changes the allowed optical modes in the medium, leading to many varied and useful properties. Some photonic crystals can completely eliminate optical modes in all directions for a specific band of wavelengths. These structures are said to exhibit a full three-dimensional (3D) photonic bandgap. The spectral emissivity is zero within a full 3D photonic bandgap. A description of photonic crystals and their properties is given by Joannopoulos et al., in Photonic Crystals: Molding the Flow of Light (1995).
The thermal radiation spectrum and, therefore, the emissivity can be altered by suitable modification of the properties of photonic crystals. Therefore, photonic crystals can potentially be used as selective emitters for TPV energy conversion. The use of photonic crystals for the control of emission of thermal radiation from an object is disclosed in copending U.S. patent application Ser. No. 09/441,221 to Lin and Fleming, which is incorporated herein by reference. Modification of the thermal radiation from a photonic structure in the infrared portion of the spectrum has been described by Lin et al. in xe2x80x9cEnhancement and suppression of thermal emission by a three-dimensional photonic crystal,xe2x80x9d Phys. Rev B62, R2243 (2000). Lin et al. fabricated a 3D xe2x80x9cLincoln-Logxe2x80x9d type silicon photonic crystal with air as the second dielectric. The silicon photonic crystal had a lattice constant of 4.2 xcexcm and a full photonic bandgap. When heated to 410xc2x0 C., the silicon photonic crystal exhibited significantly reduced emissivity in the infrared wavelength range between 10 and 16 xcexcm, indicative of suppressed emission within the photonic bandgap and enhanced emission at energies below the band edge.
Milstein et al., U.S. Pat. No. 5,601,661, discloses the use of reticulated photonic bandgap structures, including aluminum oxides, semiconductors, rare-earth oxides, and plastics, as thermophotovoltaic emitter materials. However, it is unlikely that the reticulated structures and low-dielectric-contrast materials disclosed by Milstein et al. actually behave as photonic crystals having full 3D photonic bandgaps. Therefore, the reticulated structure and materials disclosed by Milstein et al. are unlikely to achieve the true emission selectivity at high temperature necessary for efficient TPV energy conversion. In particular, Milstein et al. does not teach metallic photonic crystals having enhanced emission matched to the spectral response of a photovoltaic cell.
Therefore, a need remains for a high-temperature photonic crystal that can selectively emit radiation in a narrow band matched to the spectral response of low-bandgap photovoltaic cells, thereby enabling efficient TPV energy conversion. The present invention provides a method for TPV energy conversion based on selective emission from photonic crystals comprising metals that enable high temperature operation and a full three-dimensional photonic bandgap.
A method for thermophotovoltaic generation of electricity comprises heating a photonic crystal comprising a metal having a complex dielectric constant and at least one other lattice material having at least one other dielectric constant, whereby the photonic crystal selectively emits radiation at emission wavelengths less than an electronic bandgap wavelength of at least one photovoltaic cell, and collecting the selectively emitted radiation with the at least one photovoltaic cell, whereby the at least one photovoltaic cell converts the selectively emitted radiation to electricity. The selectively emitted radiation can be from enhanced emission below the band edge of the photonic bandgap or from defect cavity emission within the photonic bandgap. The metal can be a refractory metal, such as tungsten. The photovoltaic cell can be a low-bandgap photovoltaic cell.